Semi-quantitative analytical measurement processes of optical colorimetry are based upon the visual measurement of colorations which are formed in solutions by the addition of certain reagents. Depending upon the color reaction and the measurement range, the solutions can be of very different color intensity. Normally, the measurement takes place by color comparison against standard solutions.
The optical-visual colorimetry of weakly colored solutions is, as a rule, carried out in standard cylinders. Such weakly colored solutions occur in many fields of chemical analysis, e.g., in water analysis. The simplest form of the standard cylinder in colorimetry is the Nessler cylinder in which the level of the two solutions to be compared in so varied by pouring out that, seen from above, an identical color impression arises with regard to color shade or color intensity. In the case of the Hehner cylinder, this can be carried out more simply and more exactly by means of a small stopcock for the letting out of the solutions on the lower ends of the cylinders.
Although the principle of the standard cylinder in the optical assessment of weakly colored solutions represents a very simple and sensitive analytical measurement process, in practice it is today used less and less because of the laborious handling. This is to be attributed to the following reasons: the level regulation of the standard cylinder requires time and skill; the standard solution for the color comparison must be prepared daily or very frequently even when only one sample is to be measured, which is often the case, e.g., in the case of control analyses; an inherent coloration or turbidity of the sample solution, which is present in many fields of analysis, cannot be compensated for since the levels usually vary; the standard cylinder can easily be knocked over and requires a flat standing surface which is not present in many potential places of use.
In the case of the optical colorimetry of strongly colored solutions, the brilliance of the color effect of the low liquid columns in the standard cylinder is not sufficient because of strong shadows. Therefore, in the case of solutions with high color density, hitherto transillumination comparators have preferably been used in which the colored solutions are compared with colored synthetic resins or glasses as color standards. When suitable backgrounds, which are as neutral white as possible, are not available for the examination of the transillumination, such comparators can be backed with white cards. Because of the more expensive production, such transillumination comparators are too expensive in reagent equipment sets in which a maximum performance is sought after, having regard to the price.